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Femtosecond Telecom
Review of applications of femtosecond lasers in telecom research and development
1. 100 terabits per second transmission
using femtosecond pulses.
One Fiber, 100 Terabits:
UPC
collaboration trying to achieve 100 terabits per second transmission using
femtosecond pulses. The Ultrafast Photonics Collaboration (UPC) is an
EPSRC Interdisciplinary
Research Collaboration comprising 6 leading UK universities and 5 industrial
collaborators. The aim of UPS is to generate the technology required for the
development of the next generation of photonics with the aim of operating at
speeds in excess of 100Tb/s. Current status of UPC
research.
2. Ultrafast-laser formed waveguides in the
Telecom Spectrum
Ultrafast laser processing offers new prospects to miniaturize and
integrate highly functional photonic devices directly inside transparent
materials. Nonlinear optical interactions induce strong refractive-index changes
in sub-micron volumes that permit the generation of two and three dimensional
refractive-index structures with simple motorized translation stages. The
ultrafast interaction does not require specially prepared or photosensitive
materials and various silica-based components such as passive and active
waveguides and directional couplers have been reported. Researches Dragan Ćorić
and Peter R. Herman from Department of Electrical and Computer Engineering,
University of Toronto, Toronto, Ontario, Canada coric@ecf.utoronto.ca; hermanp@ecf.utoronto.ca
and Ravi Bhardwaj, Paul B. Corkum and David M. Rayner Steacie Institute for
Molecular Sciences, National Research Council of Canada, reported on the first
characterization of ultrafast-laser formed waveguides in the Telecom Spectrum
(1520-1620nm) and describe laser-processing windows for generating low-loss,
single and multi-mode devices, across a broad spectrum.
Article.
3. Femtosecond machining.
Article from
OE Magazine. (pdf version).
The direct machining of silica, in particular, has always been very difficult.
With the advent of commercially available fluorine-gas lasers and femtosecond
solid-state lasers, however, very-high-quality machining of silica is now
possible. Since most fibers and waveguides are made from silica, this has been
an important development.
4. Femtosecond Time-Bin Entangled Qubits
for Quantum Communication
Authors: Ivan Marcikic, Hugues de Riedmatten, Wolfgang Tittel, Valerio Scarani,
Hugo Zbinden, Nicolas Gisin
Journal-ref: Phys. Rev. A 66,062308 (2002)
Abstract (http://arxiv.org/abs/quant-ph/0205144)
We create pairs of non-degenerate time-bin entangled photons at telecom
wavelengths with ultra-short pump pulses. Entanglement is shown by performing
Bell kind tests of the Franson type with visibilities of up to 91%. As time-bin
entanglement can easily be protected from decoherence as encountered in optical
fibers, this experiment opens the road for complex quantum communication
protocols over long distances. We also investigate the creation of more than one
photon pair in a laser pulse and present a simple tool to quantify the
probability of such events to happen. (article in pdf
format)
This article was published in
lightreading.com
http://www.lightreading.com/document.asp?doc_id=3063&site=testing
A group of
The universities -- St. Andrews,
Bristol,
Glasgow,
Heriot-Watt,
and Imperial
College, London -- have already secured US$18 million (£12.5
million) of government funding for the six-year project. And now the UPC is trying to
figure out how it's going to achieve its 100-Tbit/s target.
The UPC is going to use femtosecond pulses. These short bursts of light are about 10,000 times shorter than the pulses in a 10-Gbit/s signal. Therefore, on a single channel, they would provide 10,000 times more information-carrying capacity, or 100 Tbit/s.
Of course, if it were easy to
carry traffic with such ultrashort pulses of light, folk would be doing it
already. They're not doing it because light sources, modulators, and
detectors that can operate at such phenomenal speeds don't exist. And then
there are things like dispersion to consider. Femtosecond pulses behave in a
non-linear way when they travel down an optical fiber, or any other material.
In other words, their behavior is difficult to predict.
Carrying all the information
on a single channel is one possible scenario, but it's not the only one. In its
research proposal, the UPC outlines an alternative method of exploiting
femtosecond pulses, which it terms "spectral slicing."
This has similarities with
wavelength-division multiplexing (WDM) and takes advantage of the fact that
femtosecond pulses are composed of a broad range of wavelengths. By shining the
pulses on the equivalent of a prism -- possibly an arrayed waveguide grating or
other grating -- it is possible to split each pulse into a rainbow of its
constituent wavelengths. Next, groups of wavelengths can be modulated
separately. It sounds complicated, but effectively the pulses are acting as a
single, broadband source with a total system bandwidth of, say, 200 nanometers.
"It is reasonable to
suppose that this type of source will provide the potential for ten separate
2.5 Tbit/s channels thus constituting a total transmission rate of 25
Tbit/s," the research proposal states.
Bristol University's Ian
White, who directs the systems aspect of the UPC work, points out that
individual channel speeds of 2.5 Tbit/s aren't significantly higher than what's
already been accomplished in the lab. A single-channel bit rate of 1.28 Tbit/s
has already been achieved by NTT Corp., according to a paper it
presented at the European Conference on Optical Communications
last September.
However, the key here is not
really the speed of the individual channels; it's the broadband nature of the
system. It's no good maximizing bit rate without also considering system
bandwidth, because there is a tradeoff between the two (see Essex Claims 4000-Channel DWDM ). As a
rule of thumb, the maximum attainable bit rate is roughly equal to the channel
spacing. By that reckoning, each 2.5-Tbit/s channel requires at least 20nm of
bandwidth to itself.
"That's why we're
exploring materials that will provide broad bandwidths, such as quantum dots
[tiny semiconductor particles] and polymers," says White (see Zia Laser's Not-so-Dotty Idea ).
The quantum dot work at
The universities' efforts are
being backed by commercial component vendors that have signed up as project
partners. They include Agilent Technologies Inc. (NYSE: A - message board), Kymata Ltd.,
JDS Uniphase Inc.
(Nasdaq: JDSU - message board), Marconi
Communications PLC, (Nasdaq/London: MONI - message board), Nortel Networks Corp. (NYSE/Toronto: NT - message board), Sharp Corp.,
and Vitesse
Semiconductor Corp. (Nasdaq: VTSS - message board).
– Pauline Rigby, senior
editor, Light Reading http://www.lightreading.com
Del Mar Ventures femtosecond product portfolio includes Ti:Sapphire and Cr:Forsterite oscillators and amplifiers, Femtosecond Absorption Pump – Probe Systems and Femtosecond Fluorescence Measurement Systems, as well as a variety of autocorrelators for pulse measurements, pulse pickers for pulse selection, and Faraday Isolators to protect femtosecond laser oscillators from optical feedback.
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